Such a cutter head (also called cutter) is known, for example from NL-A-9200368. In NL-A-9200368 a cutter head is described with a number of chisels which are mounted around the rotation shaft and which have at least their ends connected by means of a support construction. The support construction of the cutter head forms a conical revolving body of which a first end plane (corresponding to the ground plane) is formed by a circular support ring, and of which the second support plane (corresponding to the top plane) is formed by a hub for connection to the rotation shaft. A number of spiral-shaped ribs extend between the support ring and the hub, which are provided with the chisels. Considered in the direction of the rotation shaft, the chisels are mounted in such way to the ribs that they are oriented more or less tangentially to the rotation direction. Considered from the side, the chisels extend substantially parallel to the ground plane, or enclose a small angle with it.
In order to dredge soil with a suction dredger the cutter head is mounted to the suction dredger by means of a ladder. Such a suction dredger comprises a vessel which is anchored to the ground by means of so-called spud poles, by which a means is created to take up the strong reaction forces which occur during dredging and transfer them to the ground. In the known suction dredger the ladder forms a more or less rigid connection between the cutter head and the suction dredger. Upon dredging underwater soil the cutter head with ladder and suction pipe is lowered under water in a usually slanting direction, until it hits the bottom. In order to be able to trail the cutter head over the bottom the ladder is hauled by means of winches alternately from port side to starboard side so that a more or less circular soil path is worked. The cutter head is rotated relatively slowly (common rotation speeds of 20 to 40 rpm), as a result of which soil pieces are beaten off by the chisels at great force. By each time moving the suction dredger over a given distance and repeating the above described ladder movement, a complete soil area can be dredged.
With the known cutter head the digging operation is achieved by rotating it around the rotation shaft, so that the chisels which are oriented more or less tangentially to the rotation direction beat the soil with their tips. Because of the repeated hammer beatings of the successive chisels the soil is at last shattered into large pieces. These pieces are at least partly sucked up by the suction pipe and removed.
The known cutter head has the disadvantage that the efficiency of the dredging operation, more particularly on soils with a high UCS (Unconfined Compressive Strength), is insufficient. Considerable power is needed to achieve that the chisels beat the soil with sufficient force in order to have pieces which can be sucked up. As used herein, with efficiency is meant the volume dredged soil per unit of time and power.
At present there is also an upper limit of the rock strength that can be dredged. Beyond this limit dredging it is not possible anymore and other very expensive excavation methods such as drilling and blasting, need to be used.
From U.S. Pat. No. 4,320,925 a drilling head is known which comprises disc-shaped cutting tools. This document relates to drilling in dry conditions and does not teach the applicability of disc-shaped cutting tools for dredging underwater soils. This is reflected in the fact that the drilling head is adapted to be driven into the ground, whereas cutter heads for dredging underwater soil are hauled over the ground at the end of a ladder.